Effect of Large Downrange Obstacles on OTA Signals - Canadian TV, Computing and Home Theatre Forums

El Gran Chico · 2012-05-15, 12:05 PM

When I put my antenna up a while ago, my installer was concerned about a large condo building (34 stories) that is 2.7 km downrange (measured on Google Earth) dead center in the path to a transmitter over 100 km away.

It appears that the installer was correct. This station has been my "problem station" ever since. Last weekend I did an experiment by taking my spare antenna a block away (outside the shadow of the building) with a portable tv+STB and the station was solid, even when holding the antenna 4 feet above the ground (at the same time, my chimney mounted antenna at home wasn't able to get enough signal to lock). More evidence that supports what my installer was concerned about.

Is there a formula or rule of thumb that is a good indicator if a large object (I suspect this will usually be a building) in the direct path to a transmitter will be a problem or even fatal for reception?

By looking at some topographical maps, my antenna is about 35 meters above the elevation of the base of the building, so it's likely about 100 meters lower than the top of the building.

Without my installers professional insight, I would have (incorrectly) thought that 2.7 km would have been far enough away to not worry about it.

m81m82 · 2012-05-15, 02:54 PM

El Gran, I am sure the obstacle would affect your reception if the transmitter is behind the building. However, 1 building might allow 1 edge diffraction if the angle is not too steep. I think that my problem with CHCH (and CTS) is that the core of downtown Toronto is blocking my LOS and that signal must get through or around all that concrete and steel.

nbound-au · 2012-05-18, 05:27 AM

UHF and to a little lesser extent VHF travels in straight lines, when it hits an object such as this or the top of a hill, it can be diffracted around the obstacle. Though it will be considerably weaker (though often quite usable) when compared to the line of sight signal. A 4/8 bay bowtie antenna will generally peform well with diffracted signals when compared to a yagi antenna. Another option which may be used with or without the bowtie antenna is using a masthead amplifier. How this will effect your other signal levels is hard to predict without a signal meter on location.

It may end up that you will need to diplex a second antenna into the rest of the system, the second antenna used purely for the reception of this station.

OTOH, diffraction can cause signal levels to vary greatly over only a few metres, it may be possibly that another position on your roof has a higher signal strength. Especially if the aerial installer didnt perform a site survey.

El Gran Chico · 2012-05-18, 11:57 AM

For sure there's diffraction. In fact I've found a "good" spot for the signal on my property but it's an awful spot to erect an antenna (pretty much in the middle of the front lawn

).

I'm less concerned about a solution for me but when to properly anticipate an obstacle will be a problem when helping/diagnosing OTA issues for friends and family who are also OTAers.

For instance, if the top of the building in question from my original post was only 50 meters higher than my antenna at 2.7 km downrange, would it even be a factor? Or 50 meters higher if it was 10 km downrange?

ScaryBob · 2012-05-18, 05:44 PM

Metal contained in high rises creates completely different issues than hills. Hills generally block signals but also cause diffraction near the edges. Steel buildings generally create reflected signals that result in multipath. Trying to receive a signal "through" a steel structure will usually result in a completely garbled signal due to dozens or hundreds of reflections within the building. Best case scenario is probably a building with aluminum coated windows that reflect the signal evenly. Tall structures can be a mixed blessing. One signal with LOS here arrives with bad multipath due to a tall building about 4km away. Another signal, that would be unreceivable otherwise, is available due to a reflection off the same building. I suspect some tropo is also aided by the reflection. Now all I need to do is point a 91XG at the top of that building to see what comes in.

nbound-au · 2012-05-19, 04:26 AM

A single skyscraper at 2.7km is unlikely to result in a completely garbled signal, a whole downtown area with multiple skyscrapers and a close high-powered transmitter is another story.

I will note that I'm speaking from the perspective of an installer in an area that uses DVB-T, which has a guard interval to protect to a decent extent from multipath. And as such my experience may not directly transfer in this circumstance.

ScaryBob: With a good signal meter you may be to identify a reflection for that LOS station which would provide a usable signal.

rabbit73 · 2012-05-19, 05:31 PM

DVB-T does seem to do better with multipath than ATSC, but it requires a higher SNR, which means more transmitter power.

http://www.bbc.co.uk/rd/pubs/papers/.../crn-dvbtm.pdf

Quote:

ScaryBob: With a good signal meter you may be to identify a reflection for that LOS station which would provide a usable signal.

Yes, a signal meter or TV that shows errors or signal quality and not just strength, would be the most useful.

ScaryBob · 2012-05-19, 06:43 PM

Or a HDHomerun with Android or Windows signal meter software. That's what I use.

nbound-au · 2012-05-19, 06:53 PM

Some professional-grade signal meters actually have functions that can identify multipath signals, and help you try to minimise their effects.

2012-05-15, 12:05 PM	#1
El Gran Chico Join Date: Sep 2006 Location: Toronto/Etobicoke - Bloor/Royal York/Queensway/Islington Posts: 1,386	Effect of Large Downrange Obstacles on OTA Signals When I put my antenna up a while ago, my installer was concerned about a large condo building (34 stories) that is 2.7 km downrange (measured on Google Earth) dead center in the path to a transmitter over 100 km away. It appears that the installer was correct. This station has been my "problem station" ever since. Last weekend I did an experiment by taking my spare antenna a block away (outside the shadow of the building) with a portable tv+STB and the station was solid, even when holding the antenna 4 feet above the ground (at the same time, my chimney mounted antenna at home wasn't able to get enough signal to lock). More evidence that supports what my installer was concerned about. Is there a formula or rule of thumb that is a good indicator if a large object (I suspect this will usually be a building) in the direct path to a transmitter will be a problem or even fatal for reception? By looking at some topographical maps, my antenna is about 35 meters above the elevation of the base of the building, so it's likely about 100 meters lower than the top of the building. Without my installers professional insight, I would have (incorrectly) thought that 2.7 km would have been far enough away to not worry about it. __________________ Orig 4221, A-D C5, CM 7778, Aquos LC37D62U, TiVo Premiere, DTVpal DVR

2012-05-18, 11:57 AM	#4
El Gran Chico Join Date: Sep 2006 Location: Toronto/Etobicoke - Bloor/Royal York/Queensway/Islington Posts: 1,386	For sure there's diffraction. In fact I've found a "good" spot for the signal on my property but it's an awful spot to erect an antenna (pretty much in the middle of the front lawn ). I'm less concerned about a solution for me but when to properly anticipate an obstacle will be a problem when helping/diagnosing OTA issues for friends and family who are also OTAers. For instance, if the top of the building in question from my original post was only 50 meters higher than my antenna at 2.7 km downrange, would it even be a factor? Or 50 meters higher if it was 10 km downrange? __________________ Orig 4221, A-D C5, CM 7778, Aquos LC37D62U, TiVo Premiere, DTVpal DVR

2012-05-18, 05:44 PM	#5
ScaryBob Veteran Join Date: Feb 2009 Location: The Dandelion City Posts: 7,133	Metal contained in high rises creates completely different issues than hills. Hills generally block signals but also cause diffraction near the edges. Steel buildings generally create reflected signals that result in multipath. Trying to receive a signal "through" a steel structure will usually result in a completely garbled signal due to dozens or hundreds of reflections within the building. Best case scenario is probably a building with aluminum coated windows that reflect the signal evenly. Tall structures can be a mixed blessing. One signal with LOS here arrives with bad multipath due to a tall building about 4km away. Another signal, that would be unreceivable otherwise, is available due to a reflection off the same building. I suspect some tropo is also aided by the reflection. Now all I need to do is point a 91XG at the top of that building to see what comes in. __________________ At 20 I had a good mind. At 40 I had money. At 60 I've lost my mind and my money. Oh, to be 20 again. --Scary

2012-05-19, 06:43 PM	#8
ScaryBob Veteran Join Date: Feb 2009 Location: The Dandelion City Posts: 7,133	Or a HDHomerun with Android or Windows signal meter software. That's what I use. __________________ At 20 I had a good mind. At 40 I had money. At 60 I've lost my mind and my money. Oh, to be 20 again. --Scary

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2012-05-15, 02:54 PM	#2
m81m82 Rookie Join Date: Mar 2012 Location: East York Donlands Mortimer Posts: 8	El Gran, I am sure the obstacle would affect your reception if the transmitter is behind the building. However, 1 building might allow 1 edge diffraction if the angle is not too steep. I think that my problem with CHCH (and CTS) is that the core of downtown Toronto is blocking my LOS and that signal must get through or around all that concrete and steel.

2012-05-18, 05:27 AM	#3
nbound-au Join Date: May 2012 Location: Australia Posts: 40	UHF and to a little lesser extent VHF travels in straight lines, when it hits an object such as this or the top of a hill, it can be diffracted around the obstacle. Though it will be considerably weaker (though often quite usable) when compared to the line of sight signal. A 4/8 bay bowtie antenna will generally peform well with diffracted signals when compared to a yagi antenna. Another option which may be used with or without the bowtie antenna is using a masthead amplifier. How this will effect your other signal levels is hard to predict without a signal meter on location. It may end up that you will need to diplex a second antenna into the rest of the system, the second antenna used purely for the reception of this station. OTOH, diffraction can cause signal levels to vary greatly over only a few metres, it may be possibly that another position on your roof has a higher signal strength. Especially if the aerial installer didnt perform a site survey.

2012-05-19, 04:26 AM	#6
nbound-au Join Date: May 2012 Location: Australia Posts: 40	A single skyscraper at 2.7km is unlikely to result in a completely garbled signal, a whole downtown area with multiple skyscrapers and a close high-powered transmitter is another story. I will note that I'm speaking from the perspective of an installer in an area that uses DVB-T, which has a guard interval to protect to a decent extent from multipath. And as such my experience may not directly transfer in this circumstance. ScaryBob: With a good signal meter you may be to identify a reflection for that LOS station which would provide a usable signal.

2012-05-19, 06:53 PM	#9
nbound-au Join Date: May 2012 Location: Australia Posts: 40	Some professional-grade signal meters actually have functions that can identify multipath signals, and help you try to minimise their effects.